Perspectives on metal-catalysed syntheses of carbolines (α/β/γ/δ): recent advances

Pyridine-fused indole derivatives, commonly known as carbolines, have emerged as fascinating and privileged heterocyclic scaffolds because of their immense applications in medicinal chemistry, chemical biology and materials sciences. These are primarily divided into four sub-groups, namely, α-, β-, γ- and δ-carbolines, which are widely prevalent in naturally occurring alkaloids, drugs, bioactive compounds, OLEDs, PLEDs, and bipolar host materials, among others. The inherent potential of these compounds has inspired researchers to develop a myriad of methods for their synthesis. These are usually prepared through fusion either of a new pyrrole ring generated from diaryl substrates, or of a pyridine ring preformed or generated in situ from indole substrates. In addition, simultaneous formation of both pyrrole and pyridine rings is also adopted in the construction of carbolines. Among these methods, metal-catalysed synthesis has made remarkable advances in the recent past due to the unique reactivity and the flexibility in using different substrates. This review intends to provide a brief yet comprehensive account of these methods for the synthesis of carboline isomers, which have mostly appeared in the literature in the last two and a half decades, along with the mechanistic pathways involved therein.